Vomiting during pregnancy has been described since 2,000 B.C., but few studies have attempted to describe its epidemiology. First trimester registrants in the Collaborative Perinatal Project were screened for the presence of vomiting. Vomiting was more common in primigravidas, young women, heavy women, non-smokers and women with less education. The absence of vomiting placed a woman at increased risk of fetal loss. There was a modest protective effect on preterm delivery, and no effect on the incidence of low birth weight. Adjustment for confounders by multiple logistic regression confirmed these associations. The effect of vomiting in the absence of use of antiemetic drugs on the incidence of congenital malformations was also examined. This analysis used women who registered during the first 20 weeks of pregnancy. Classification of malformations began with the data of Myianthopoulos and Chung, but was modified to reflect current concepts of the pathogenesis of malformations. Antiemetic drugs were defined as those classified by Heinonen, Sloane and Shapiro as "antinausiants, antihistamines and phenothiazines" and were also listed in the 1965 Physicians Desk Reference as being indicated for nausea, motion sickness, hyperemesis or vomiting. It was found that vomiting was unassociated with either major malformations, deformations, hernias or minor malformations. Adjustment for race, maternal age, gravidity, infant sex, study center and antiemetic use did not substantially alter the odds ratios.